The present invention relates generally to managing firing sequence phase transitions during skip fire operation of an engine. The invention is also useful in applications where it is desirable to transition from dynamic skip fire engine control into fixed cylinder-based firing patterns.
Skip fire engine control is understood to offer a number of benefits including the potential of increased fuel efficiency. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously during certain low-load operating conditions and remain deactivated as long as the engine remains in the same displacement mode. Thus, the sequence of specific cylinders firings will always be exactly the same for each engine cycle during operation in any particular variable displacement mode (so long as the engine maintains the same displacement), whereas that is often not the case during skip fire operation. For example, an 8-cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it operates using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed displacement modes.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns. The Applicants, has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106; 9,399,964, and others describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a dynamic skip fire operational mode. Each of these patents and patent applications is incorporated herein by reference.
Some firing fractions used while operating in a dynamic skip fire mode will result in the same cylinders being fired each engine cycle. When this occurs, it may sometimes be desirable to control which specific cylinders are being fired. The present application describes techniques that can be used to manage the phase of a firing sequence and is particularly useful in conjunction with dynamic skip fire control.